This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. As part of ACERT's standing service commitment, ACERT staff members have undertaken engineering analysis and upgrade consultation to improve an existing CW ESR instruments such as those belonging to the ACERT center and also to a separate Cornell research group headed by Dr. Gerald Feigenson. The instruments under review are outdated Varian c.w. ESR spectrometer systems (E-12, E-4) which exhibit S/N and AFC limitations adversely affecting ongoing imaging work and the study of weak-sample spectra. We have engineered a straightforward front-end (base band) receiver redesign which will improve the signal detection sensitivity and, hence, the utility of these spectrometers. For c.w. imaging work, novel methods of field stabilization have been implemented to augment the existing AFC system stability. These upgrade efforts are strongly related to our major service role of assisting ESR researchers to develop, in their own laboratories, the instrumentation needed to perform some of the unique experiments that we have developed and are currently refining. It is anticipated that the performance upgrades that we have created and which are in continuing development will serve as a generally useful and applicable upgrade retrofit for a generation of older CW spectrometers still operational in many laboratories.